Solid-state organic light emitting diode (OLED) image display devices are of great interest as a superior flat-panel digital display device and in solid-state lighting applications. These OLED devices utilize current passing through thin films of organic material to generate light. The color of light emitted and the efficiency of the energy conversion from current to light are determined by the composition of the organic thin-film material.
The OLED devices are not perfectly efficient and produce heat as a by-product of the energy conversion from current to light. As is well known, OLED materials degrade more rapidly in the presence of heat. Moreover, the thin-film transistors typically used to control OLED devices are sensitive to heat. Efficiency of the OLED materials can also be affected by temperature. Hence, there is a need to understand the temperature of an OLED device so that appropriate controls or corrections can be implemented to maximize the performance of an OLED device.
Thin-film temperature sensors are known in the prior art. For example, U.S. Pat. No. 6,774,883 entitled “Electro-optical display device with temperature detection and voltage correction” issued 20040810 describes a display device provided with a thin-film digital thermometer for sensing the temperature of the supporting plate of an electro-optical medium, for example, a liquid crystal display medium. U.S. Pat. No. 5,775,811 entitled “Temperature sensor system using a micro-crystalline semiconductor thin film” issued 19980707 describes a method for manufacturing a thin-film temperature-sensitive device. It is also known to apply such sensors to displays and LCDs. US20040150590 entitled “OLED display with aging compensation” by Cok et al published 20040805 describes an OLED display that includes a plurality of light emitting elements and a temperature measuring device located on the substrate.
While these methods are useful, the disclosures only describe measuring the temperature of a device at a single location. Applicants have demonstrated that OLED devices in operation can have a variable temperature across the substrate so that a measurement at a single location on the substrate may not correspond to the temperature elsewhere or to the average temperature. Indeed, given that the sensor is likely to be located at the periphery of the display, it is likely to be cooler than other locations on the display. Moreover, as described an additional circuit must be provided, reducing yields and potentially complicating the design layout of the OLED display.
There is a need therefore for an improved OLED device and method for the detection of temperature within an OLED device.